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A single crystal of GdBaCo 2 O 5.47 (2) has been studied by means of X-ray diffraction. Appearance of superstructure reflections at T = 341.5(7) K gives an evidence of continuous transition to the phase with unit cell doubled along the shortest edge a 1 . Critical exponent for the order parameter is found to be 0.33(1) = β .The superstructure reflections are about 2-4 orders of magnitude weaker than the basic ones. Their systematic extinction indicates the crystal symmetry change from to . The integrated intensities allow to calculate displacements of atoms from the positions in the high-temperature phase. The cobalt-ligand distances in the ordered phase are discussed in terms of the spin-state/orbital ordering of Co Pmmm Pmma 3+ ions. PACS numbers: 61.10.+Nz; 61.50.Ks; 61.66.Fn Since discovery of giant magnetoresistance in the oxygen-deficient layered perovskites RBaCo 2 O 5+x , where R is a rare earth [1], these materials attract high interest. Their orthorhombic structure at x ≈ 0.5 with the unit cell a 1 ≈ a p , a 2 ≈ 2a p , a 3 ≈ 2a p , where a p is parameter of the pseudocubic perovskite cell, is usually described by the Pmmm space group [2-5]. One can imagine the structure as a sequence of stacking plains [CoO 2 ][BaO][CoO 2 ][RO x ] along [0,0,1], which results in two types of the cobalt environment: CoO 5 pyramids and CoO 6 . octahedra. The nominal valence of cobalt at x = 0.5 is 3+. It is known [6] that the Co 3+ ion has a non-magnetic, or low-spin ground state (LS, ) as well as two excited states, the intermediate-spin (IS, ) and the high-spin (HS, ) ones. The energy differences are small enough to gain the excited states by the thermal fluctuations or due to the lattice change, which results in spin state transitions [7]. A metal-insulator (MI) transition has been also found, with the transition temperature for GdBaCo 0 6 2 g g e t 1 5 2 g g e t 2 4 2 g g e t 2 O 5+x 350 K < T MI < 370 K being dependent on the oxygen content of 0.4 < x < 0.47 [1, 2 8-10]. The transition is of the first order, which is indicated by a hysteresis of 8 K in resistivity for TbBaCo 2 O 5.4 [8].In spite of numerous studies of these materials, neither Co 3+ spin state nor nature of the MI transition is finally established. A spin-state transition coupled with the orbital degrees of freedom is suggested to be a driving force for the MI transition. The distribution of the IS e g orbitals (3x 2 -r 2 ) in pyramids and (3y 2 -r 2 ) in octahedral sites on cooling has been suggested as an origin of transition on the basis of structural studies of TbBaCo 2 O 5.5 [11]. On the other hand, it has been concluded that the transition to metallic phase in GdBaCo 2 O 5.5 is due to excitation of the LS-state electrons into e g band of the Co HS-state in octahedra; with Co in pyramids having IS both sides of T MI [9]. This conclusion has been made because of octahedron expansion of about 0.012(4) Å and simultaneous pyramid shrinking. The spin-state as well as the orbital ordering among one type of coordinating polyhedra was considered in a num...

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Magnetization studies have been carried out on a single crystal of TbBaCo 2 O 5.5 in the temperature range 2-350 K and magnetic fields up to 50 kOe. Several spontaneous magnetic phase transitions have been observed and analysed in both Co (T C = 277 K, T N1 = 210 K) and Tb (T N2 = 3.44 K) subsystems. It is shown that the spontaneous first-order phase transition at T N1 from an antiferromagnetic to a weak ferromagnetic state is accompanied by an intermediate state. The properties of this state are strongly determined by the thermo-magnetic history of the sample. Metamagnetic type transitions have been observed at 140 K < T < 190 K for the Co subsystem and at T < T N2 for the Tb subsystem. The analysis of the magnetic properties of TbBaCo 2 O 5.5 points to the strongly anisotropic character of the Tb and Co subsystems. The Tb subsystem was shown to be of the Ising type. For this subsystem, the exchange interactions between nearest neighbours and next-nearest neighbours were determined. The observed properties of the Co subsystem also point to the Ising character of the Co ions. The effect of the twinning structure on magnetization processes of the weak ferromagnetic phase, especially hysteresis curves, is discussed.